Method of insulating a case of a solid propellant rocket motor

ABSTRACT

This method permits manufacturing EPDM rocket motor insulation in which carbon fibers are dispersed and immobilized in the EPDM polymeric matrix but are not excessively fractured or fragmentized, i.e., broken into smaller fragments, when encountering degrees of shear necessary to homogeneously or otherwise distribute or disperse the carbon fibers in the EPDM polymeric matrix. The method is substantially solvent free and is performed via distributive/reduced shear mixing to distribute the fragile carbon fibers into a rubber matrix without excessive damage. According to one embodiment, at least about 50% of the elastomer composition introduced into the mixing apparatus is liquid EPDM terpolymer having sufficiently low molecular weight and high diene content to permit dispersion of the carbon fibers in the EPDM without substantial fragmentation of the fibers. According to another embodiment, mixing takes place in a kneader capable of rotating a screw having a discontinuous screw thread about the screw axis while superimposing an axially reciprocating stroke to the screw. The kneader imparts low shear distributive mixing of the carbon fibers in the EPDM terpolymer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 09/747,192,filed Dec. 21, 2000, now U.S. Pat. No. 6,893,597, issued May 17, 2005,which claims the benefit of U.S. Provisional Patent Application Ser. No.60/171,619 filed in the U.S. Patent & Trademark Office on Dec. 23, 1999,now abandoned, the complete disclosure of which is incorporated hereinby reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided by the terms of F04611-97-C-0053to the Air Force Rocket Laboratory.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed to a process for making elastomer-basedinsulation for rocket motors and, in particular, to a process in whichfragile carbon fibers are mixed with and preferably homogeneouslydispersed in ethylene propylene diene monomer (EPDM) without requiringthe use of a volatile solvent for dissolution of EPDM during fiberincorporation. The insulation of this invention is especially useful forplacement in the nozzle or case, including between a solid propellantgrain and a rocket motor case for protecting the case from hightemperatures experienced during burning of solid propellant grains.

2. State of the Art

Solid rocket motors typically include an outer case or shell housing asolid propellant grain. The rocket motor case is conventionallymanufactured from a rigid, yet durable, material such as steel orfilament-wound composite. The propellant is housed within the case andis formulated from a composition designed to undergo combustion andthereby produce the requisite thrust for attaining rocket motorpropulsion.

During operation, a heat insulating layer (insulation) protects therocket motor case from heat and particle streams generated by thecombusting propellant. Typically, the insulation is bonded to the innersurface of the case and is generally fabricated from a compositioncapable of withstanding the high temperature gases produced when thepropellant grain bums. A liner layer (liner) functions to bond thepropellant grain to the insulating layer and to any noninsulatedportions of the case, as well as to inhibit interfacial burning. Linercompositions are generally known to those skilled in the art. Anexemplary liner composition and process of applying the same to a caseis disclosed in U.S. Pat. No. 5,767,221, the complete disclosure ofwhich is incorporated herein by reference to the extent that it iscompatible with this specification.

The combustion of solid rocket propellant generates extreme conditionswithin the case of the rocket motor. For example, temperatures insidethe rocket motor case typically reach 2,760° C. (5,000° F.) and interiorpressures may exceed 1,500 psi. These factors combine to create a highdegree of turbulence within the rocket motor case. In addition,particles are typically entrained in the gases produced duringpropellant combustion. Under the turbulent environment, these entrainedparticles can erode the rocket motor insulation. If the insulating layerand liner are pierced during rocket motor operation, the casing issusceptible to melting or degradation, which can result in failure ofthe rocket motor. Thus, it is crucial that insulation compositionswithstand the extreme conditions experienced during propellantcombustion and protect the case from the burning propellant. It is alsocrucial that insulation compositions possess acceptable shelf lifecharacteristics such that they remain sufficiently pliable, withoutbecoming fully cured, until used in application to the rocket motorcasing. This requirement is essential because the production of a givenlot of insulation may have to wait in storage for a number of monthsprior to use. Typically, the insulation may be stored in large rolls inan uncured, or at most, a partially cured, state until ready for use. Anumber of curing agents are well known and are conventionally employedbut still must be compatible with the overall EPDM formulation to permitsatisfactory shelf life. This in turn requires a balancing of curingagent activity.

In the past, attempts at producing insulating materials that wouldprotect the rocket motor case focused on filled and unfilled rubbers andplastics, such as phenolic resins, epoxy resins, high temperaturemelamine-formaldehyde coatings, ceramics, polyester resins, and thelike. These plastics, however, crack and/or blister as a result of therapid temperature and pressure fluctuations experienced duringcombustion.

Elastomeric compositions have also been used as rocket motor insulationmaterials in a large number of rocket motors. The elastomericcompositions have been selected because their mechanical, thermal, andablative properties are particularly suited for rocket motorapplications. However, the ablative properties of elastomers are ofteninadequate for rocket motor operation. For example, insulation, whetherthermosetting or thermoplastic, is characterized by relatively higherosion rates unless reinforced with a suitable filler. The criticalityof avoiding such high erosion rates is demonstrated by the severity andmagnitude of the risk of failure due to erosion. Most insulation is, ofnecessity, “man-rated,” in the sense that a catastrophic failure canresult in the loss of human life whether the rocket motor is used as abooster for launch of a space shuttle or is carried tacticallyunderneath the wing of an attack aircraft. The monetary cost of failurein satellite launches is well publicized and can run into the hundredsof millions of dollars.

In order to improve the ablative properties of elastomeric compositions,it has been proposed to reinforce the elastomeric compositions withfillers, such as organic-based fibers or carbon fibers. For instance, anexemplary carbon fiber-filled rocket motor insulation composed of solidNORDEL® 1040 as the primary terpolymer is commonly known in the industryas the STW4-2868 thermal insulation and has the following composition:

TABLE A STW4-2868 THERMAL INSULATION FORMULATION (carbon fiber; parts byweight) Parts by Ingredient Function Weight NORDEL ® 1040 Primary EPDMterpolymer base 80 Neoprene FB Secondary polymer base 20 Zinc oxideActivator 5 Sulfur Curative 1 HAF carbon black Pigment 1 MBT Accelerator1 AGERITE ® Resin D Antioxidant 2 AGERITE ® HPS Antioxidant 1 TELLURAC ®Accelerator 0.50 SULFADS ® Accelerator 0.75 VCM carbon fibers Filler 41Total Parts by Weight 153.25

Although organic-based fibers can be dispersed within the EPDM withouttoo much difficulty, the homogeneous dispersion of carbon fibers in anelastomeric composition presents a difficult processing problem. Themixing process is complicated by the fragility of the carbon fibers.Mixing of carbon fibers into a solid elastomer under high shearphysically deteriorates the carbon fibers into smaller particles orshreds, thereby negating the advantageous physical attributes that thecarbon fibers would otherwise have contributed to the insulation.

Conventionally, the problem of carbon fiber fragility has been addressedby dissolving the elastomer into a solution with an appropriate organicsolvent to lower the viscosity of the elastomer or elastomer mixture.Suitable solvents include, by way of example, hydrocarbons such ashexanes, heptanes, and/or cyclohexane. The frangible graphitized carbonfibers can then be mixed with the solution in, for example, asigma-blade mixer without significant breakage of or damage to thecarbon fibers. The material is then sheeted out and the solvent isallowed to evaporate at ambient atmosphere or in an oven.

The use of solvent in this processing technique presents severaldrawbacks. For example, solvent processing techniques, such as thoseconventionally used to disperse carbon fibers in EPDM rubber, arerelatively expensive. Material costs are increased by the use ofsolvents, as are processing costs, since additional workers andequipment are required to handle and process the solvents. Further,considerable costs and worker safety issues are associated with thedisposal of hazardous volatile organic solvents.

Thus, although it has been long recognized that carbon fiber-filled EPDMis an excellent candidate for rocket motor insulation, a low cost andnonhazardous solvent-free synthesis route that produces EPDM insulationhaving carbon fibers homogeneously dispersed therein, but without beingsubject to significant breakage or damage would be desirable.

BRIEF SUMMARY OF THE INVENTION

Therefore, a method of manufacturing ethylene propylene diene monomer(EPDM) rocket motor insulation in which carbon fibers are dispersed andimmobilized in the EPDM polymeric matrix, but are not excessivelyfractured or fragmentized, i.e., broken into smaller fragments, whenencountering degrees of shear necessary to homogeneously or otherwisedistribute or disperse the carbon fibers in the EPDM polymeric matrix isprovided.

The method of the present invention is a substantially solvent-freemethod in which the insulation is manufactured via distributive/reducedshear mixing to distribute the fragile carbon fibers into a rubbermatrix without excessive damage.

In accordance with one embodiment of this substantially solvent-freemethod, the elastomer composition comprises carbon fibers and EPDMterpolymer, at least 50 wt % of which is introduced as an ingredientinto the mixing apparatus as liquid EPDM terpolymer having asufficiently low molecular weight and high diene content to permitdispersion of the carbon fibers in the EPDM without substantialfragmentation of the fibers. As referred to herein, “liquid EPDM” meansEPDM terpolymer that is flowable at room temperature. Suitable mixingapparatuses for this embodiment include sigma-blade and vertical-blademixers. Certain kneaders, such as discussed below in connection withanother embodiment of the inventive method, capable of superimposing arotational and axial mixing motion to the carbon fibers can also beused.

In accordance with another embodiment of the invention, the elastomercomposition is prepared, optionally under substantially solvent-freeconditions with little or no liquid EPDM terpolymer, by use of a kneadercapable of rotating a screw having a discontinuous screw thread aboutthe screw axis while superimposing an axially reciprocating stroke tothe screw. This kneader imparts low shear distributive mixing of thecarbon fibers in the EPDM terpolymer. The kneader used in thisembodiment is especially suitable where little or no liquid EPDMingredient and no volatile solvent are included in the formulation.

As referred to herein, carbon fibers are fibers having been subject toat least substantial graphitization or carbonization, and preferablyhave about 98 wt % or more carbon content.

Other aspects and advantages of the invention will be more apparent tothose skilled in the art upon reading the detailed description andappended claims, which when read in conjunction with the accompanyingdrawings, explain the principles of this invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The accompanying drawings serve to elucidate the principles of thisinvention. In such drawings:

FIG. 1A is a schematic cross-sectional view of a rocket motor assemblyin which the insulation is provided;

FIG. 1B is an enlarged schematic cross-sectional view of the areaencircled and labeled “SEE FIG. 1B” in FIG. 1A;

FIG. 2 is a schematic cross-sectional view of a kneading apparatussuited for use with this invention;

FIG. 3 is a cross-sectional view taken along sectional line III-III inFIG. 2;

FIG. 4 is a schematic view of an axial segment of a discontinuous screwbarrel of the kneading apparatus of FIG. 2, with the axial segmentsection being projected onto a flat plane for explanatory purposes;

FIG. 5 is the schematic view of FIG. 4, with kneading pins of thekneading apparatus being superimposed onto the illustrated axialsegment;

FIG. 6 is the schematic view of FIG. 5, showing the paths of relativemovement of selected ones of the kneading pins relative to thediscontinuous screw barrel and, in particular, relative to the screwflights of the barrel; and

FIG. 7 is a schematic, cross-sectional view of a char motor used intesting examples discussed hereinbelow.

DETAILED DESCRIPTION OF THE INVENTION

The present insulation compositions 10, when in a cured state, areespecially suited for disposal on the interior surface of the rocketmotor case 12, as shown in FIGS. 1A and 1B. Typically, a liner 14 isinterposed between the insulation composition 10 and propellant 16. Theinsulation composition 10 and the liner 14 serve to protect the rocketmotor case 12 from the extreme conditions produced by the propellant 16as it undergoes combustion reactions and is exhausted through nozzleassembly 18. Methods for loading a rocket motor case 12 with theinsulation composition 10, the liner 14, and the propellant 16 are knownto those skilled in the art, and can be readily adapted within the skillof the art without undue experimentation to incorporate the insulationcomposition 10 of this invention.

Unlike conventional techniques that make use of a solvent within amixing apparatus to achieve adequate distribution of carbon fibers insolid EPDM ingredients without significant fiber fragmentation, themethod of this invention achieves distribution of carbon fibers in anEPDM matrix under solvent-free conditions, or at least substantiallysolvent-free conditions. As referred to herein, “substantiallysolvent-free” means that the process is performed with a sufficientlysmall amount of volatile solvent that, even if the volatile solvent isnot removed during manufacture of the insulation, the volatile solventwill not be present in a sufficient amount to violate applicableenvironmental or safety regulations during manufacture, rocket motorstorage, or rocket motor operation due to volatilizing of the solvent.Generally, the term “substantially solvent-free” preferably means notmore than about 5 wt % of volatile solvent based on the dry ingredientsof the insulation. Preferably, the process is conducted completely freeof volatile solvent.

In accordance with a first embodiment of this invention, liquid EPDM isused as a significant portion of the EPDM ingredients introduced intothe mixing apparatus. The amount of liquid EPDM ingredient used toensure adequate distribution of the fibers, without accompanyingexcessive fragmentation of the fibers, depends upon the mixing apparatusused. Generally, where a conventional mixer known in the insulationindustry, such as a sigma-blade mixer, is used to disperse the carbonfibers within the EPDM matrix, the insulation composition preferablycontains at least about 50% by weight, and more preferably at leastabout 90% by weight, liquid EPDM as an ingredient, based on the totalweight of the EPDM (i.e., both the solid and liquid EPDM ingredients).Where a vertical blade mixer is used to disperse the carbon fiberswithin the EPDM matrix, the insulation composition preferably containsslightly more liquid EPDM, such as at least about 90% by weight, andmore preferably at least about 95% by weight liquid EPDM as aningredient, based on the total weight of the EPDM (i.e., both the solidand liquid EPDM ingredients). Where a kneader such as the oneillustrated in FIGS. 2-6 is used, even less of the liquid EPDM (or evenno liquid EPDM, as detailed in the second embodiment below) is requiredto obtain homogeneous dispersion of the fibers without excessivefragmentation, i.e., all of the EPDM can be in a solid state whenintroduced into the kneader.

Generally, the EPDM, i.e., both the solid and liquid ingredients,comprises from about 35 wt % to about 90 wt %, and still more preferablyfrom about 45 wt % to about 75 wt %, of the total weight of the rocketmotor insulation. The EPDM terpolymer can be formed from 1,4-hexadiene,dicyclopentadiene, and/or an alkylidene norbornene, such as ethylidenenorbornene (ENB), as the diene component. Suitable commerciallyavailable liquid EPDM terpolymers are TRILENE® 67A and TRILENE® 77,available through Uniroyal Chemical Company of Middlebury, Conn. It isnoted, however, that a portion or all of the liquid EPDM can besubstituted for another liquid polymer ingredient, such as liquidpolyurethanes, so long as the substituted liquid polymer ingredientobtains the same distributive function with regard to the carbon fiberswithout excessive fragmentation. Suitable solid EPDM terpolymers havinga 1,4-hexadiene component for use in this invention include NORDEL®1040, NORDEL® 2522, and NORDEL® 2722E, made by DuPont Dow Elastomers ofWilmington, Del. Suitable solid EPDM terpolymers having an ENB dienecomponent for use in this invention include, without limitation, and asstated above, KELTAN® 4506, KELTAN® 1446A, KELTAN® 2308, each of whichis available from DSM Elastomers of the Netherlands, and NORDEL® IP 4520and NORDEL® IP 4640, both of which are and continue to be available fromDuPont Dow Elastomers. agents for cross-linking and/or chain extendingpolymers or polymer precursors (e.g., prepolymers). Suitable insolublesulfur curing agents are AKROSPERSE® IS-70 from Akrochem Corporation ofAkron, Ohio, and CRYSTEX® OT-20 available through Charles H. Haynes,Inc. Other forms of elemental sulfur can also be used. Suitable peroxidecuring agents include dicumyl peroxide,2,5-dimethyl-2,5-bis-(t-butylperoxy)hexane,2,5-dimethyl-2,5-bis-(benzoylperoxy)hexane,2,5-dimethyl-2,5-di(t-butylperoxy)-3-hexane,n-butyl-4,4-bis-(t-butylperoxyl)valerate,4,4′-methyl-bis-(cyclohexylamine)carbomate,1,1-bis-(t-butylperoxy)-3,3,5-trimethylcyclohexane,α,α′-bis-(t-butylperoxy)-diisopropylbenzene,2,5-dimethyl-2,5-bis-(t-butylperoxy)hexyne-3, and t-butyl perbenzoate. Acommercially available peroxide is available under the trade nameDI-CUP® 40KE, which comprises about 40% dicumyl peroxide on a claycarrier (the clay carrier is available from Burgess Pigment Company ofSandersville, Ga.). Another suitable curing agent (besides sulfur andperoxide curing agents) is bromomethyl alkylated phenolic resin,available as SP-1056 from Schenectady International, Inc. ofSchenectady, N.Y.

In typical formulations, the curing agent comprises from about 0.5 phrto about 8 phr and, more preferably, about 2 phr to about 5 phr. Asreferred to herein and generally accepted in the art, “phr” means partsby weight per one hundred parts by weight polymer.

The curing package preferably also includes at least one phosphate cureaccelerator. In the case of a sulfur curing agent, the accelerator canbe, by way of example, RHENOCURE® AP-5, RHENOCURE® AP-7, RHENOCURE®AP-3, RHENOCURE® ZADT/G, and RHENOCURE® S/G, which are available fromRhein Chemie Corporation of Trenton, N.J., and Accelerator VS, availablefrom Akrochem Corporation. Additional cure accelerators that may be usedin combination with the phosphate cure accelerator include butyl zimate;benzothiazyl disulfide (commercially known as ALTAX®);dithiocarbamate-containing blends (such as AKROFORM® DELTA P.M. fromAkrochem Corporation); and sulfides such as dipentamethylenethiuramhexasulfide (such as SULFADS® from R.T. Vanderbilt Company, Inc. ofNorwalk, Conn.). While the use of Accelerator VS was initiallyunacceptable in some formulations because of the foul odor problem itgenerated, it has also been now found that such formulations can beprepared with no significant odor if about 1.0 phr magnesium oxide isadded thereto.

Suitable cure activators for the curing package include metal oxides,such as zinc oxide (e.g., TZFD-88p from Rhein Chemie Corp.), magnesiumoxide (e.g., ELASTOMAG® 170 from Morton Chemical Co.), and stearic acid(including palmitic acid), which is available from Harwick StandardDistribution Corp. of Akron, Ohio.

The carbon fibers are fibers which have been subjected to at leastpartial graphitization or carbonization and preferably have about 98 wt% or more carbon content. The carbon fibers should have lengths suitablefor distribution in mixing equipment. Generally, the carbon fibers arepreferably noncontinuous and not less than about 1/16 of an inch inlength and not more than about 6 inches in length, although these rangesare not exhaustive as to the scope of the invention. Carbon fibers aresupplied commercially by several companies, including FORTAFIL® fibers(e.g., FORTAFIL® 140 and FORTAFIL® 144) from Akzo Nobel of Knoxville,Tenn., carbon fibers available from Amoco of Charleston, S.C., andPANEX® 33 (¼″×8″ or ¼″×15″), supplied by Zoltek Corporation of St.Louis, Generally, the carbon fibers are present in an amount of fromabout 2 wt % to about 50 wt %, more preferably from about 10 wt % toabout 30 wt %, based on the total weight of the insulation. The amountof carbon fibers will generally vary depending on the presence of otheringredients, such as char-forming agents, especially phosphate fireretardants, which supplement the carbon fibers by imparting desiredphysical properties to the insulation.

The carbon fibers can be used alone or in combination with othermaterials affecting the ablative and mechanical properties of theinsulation. By way of example, suitable materials includepolybenzoxazole fibers, polybenzimidazole fibers, aramid fibers, ironoxide, milled glass, silica, ceramic clay, and the like. Suitable silicaparticles include HI-SIL® 233 available from PPG Industries, Inc. ofLake Charles, La., and hydrophobized silica particles available fromCabot Corporation of Boston, Mass., as CAB-O-SIL® TS-610, CAB-O-SIL®TG-308F, CAB-O-SIL® TG-720, CAB-O-SIL® TS-500, CAB-O-SIL® TS-530, andCAB-O-SIL® TG-810G; Degussa AG of Germany as AEROSIL® R972, AEROSIL®R974, AEROSIL® R812, AEROSIL® R812S, AEROSIL® R711, AEROSIL® R 504,AEROSIL® R8200, AEROSIL® R805, AEROSIL® R816, AEROSIL® R711, andAEROSIL® R104; and Tulco Inc. of Ayer, Mass., as TULLANOX™ 500.

Suitable additives that may be added as required or desired include oneor more of the following, in various combinations: antioxidants, flameretardants, tackifiers, plasticizers, processing aids, carbon black,pigments, and bonding agents.

Representative antioxidants for improving the longevity of the curedelastomer include, by way of example, diphenylamine reacted withacetone, available as BLE®-25 Liquid from Uniroyal Chemical Company; amixture of mono-, di-,and tri-styrenated phenols, available as AGERITE®SPAR from B.F. Goodrich Chemical Ltd. of Australia. Other suitableantioxidants include polymerized 1,2-dihydro-2,2,4-trimethylquinoline(AGERITE® RESIN D) and mixed octylated diphenylamines (AGERITE®STATLITES), each of which is available from R.T. Vanderbilt Co., Inc.

Fillers that function as flame retardants, or char-forming additives,can be used, if desired, in lesser amounts than most other additives,which makes it easier to formulate the insulation with good mechanicalproperties. Both inorganic and organic flame retardants are expected tobe useful in the present invention. Examples of organic flame retardantsinclude: chlorinated hydrocarbon, available as DECHLORANE®, incombination with antimony oxide (optionally with diisodecyl phthalate(DIDP)) or hydrated alumina (such as Hydral 710 aluminum trihydrate);melamine cyanurate; phosphate and phosphate derivatives, available asPHOS-CHEK® P/30 (ammonium polyphosphate) produced by Monsanto ChemicalCompany of St. Louis, Mo., which can be used alone or in combinationwith pentaerythritol; DECHLORANE PLUS® 25 from Occidental ChemicalCorporation of Niagara Falls, N.Y.; and silicone resin, such as DC4-7051available through Dow Coming. An example of an inorganic flame retardantis zinc-borate, such as FIREBRAKE® ZB from U.S. Borax Inc. of Valencia,Calif.

Examples of suitable tackifiers are WINGTACK® 95 and AKROCHEM® P-133.Other ingredients, such as pigments and extruder processing aids (e.g.,ARMEEN® 18-D) well known in the art and/or suitable for use in rocketmotor thermal insulation applications and extruder techniques, areintended to be included within the scope of the present invention. Asuitable modifying elastomer is chlorosulfonated polyethylene, such asHYPALON®-20 available from DuPont Dow Elastomers. Nonvolatileplasticizers, such as hydrocarbon oil, can also be used.

The casting of the inventive insulation into a case and curing of theinventive insulation may be performed in accordance with techniquesknown in the art. As referred to herein and in the appended claims, theinventive composition can be, inter alia, either applied by casting intoa rocket motor case and then cured, or cured, optionally cut intoappropriate geometry and size, and then applied into the rocket motorcase.

Referring now more particularly to FIGS. 2-6, the kneader in accordancewith a preferred embodiment of this invention is a BUSS® Kneaderavailable through Buss Compounding Systems, AG, a plant engineeringgroup of Georg Fischer Plant Engineering. A representative BUSS® Kneaderbrand kneader is model MDK/E-46. This kneader is commercially availableand is currently believed to have been used in the past in various otherindustries, including the following: construction; electrical andelectronic component parts; automotive parts; chemicals; houseappliances; foodstuffs, packaging, and consumer goods. Another similarkneader is available from B&P Process Equipment & Systems.

The BUSS® Kneader brand kneader has a housing module (or barrel) 20defining a chamber 22. A plurality of additional housing modules (notshown) having respective chambers can be united together to provide anextended chamber. The housing module 20 can be equipped with a jacket orinternal fluid passages for heating. In order to allow for ease inmaintenance and operation, the housing module 20 can be a split-barrelarrangement to allow opening of the barrel 20 along its length, therebyfacilitating access to the chamber 22.

In the illustrated embodiment, a single rotatable screw 24 is receivedin the chamber 22. Generally, the screw 24 is from about 30 mm to about200 mm in diameter and has a length-to-diameter (L:D) ratio of fromabout 8:1 to about 20:1, although this invention is not so limited,given the flexibility of uniting a desired number of housing modules 20.

As shown in FIGS. 2-6, the periphery of the screw 24 has a plurality ofscrew flights 30. The screw flights 30 each have a rhombic configurationin the illustrated embodiment, although the present invention is notthereby limited in scope. As best shown in FIG. 4, the screw flights 30are arranged relative to each other to provide a plurality of screwflight columns 32. For each of these screw flight columns 32, therespective screw flights 30 thereof are aligned along the longitudinalaxis of the screw 24, yet spaced from each other by an axial distance.In a preferred embodiment, the screw 24 has three screw flight columns32 a, 32 b, and 32 c. The circumferential centers C of the screw flights30 of screw flight column 32 a are positioned about the circumference atintervals of 120° from the circumferential centers of the screw flights30 of screw flight columns 32 b and 32 c. Defined between each of theadjacent columns 32 a, 32 b, and 32 c are gaps 34 a, 34 b, and 34 c.Whereas the screw of a conventional single-screw extruder has acontinuous spiral or helical screw face extending along its length, thescrew 24 of the illustrated embodiment has a discontinuous screw face,with the spiral or helical path of the screw face being interpreted bythe gaps 34 a-c (collectively referred to as “gaps 34”).

The housing module 20 has kneading pins (also referred to as kneadingteeth) 40, which in the illustrated embodiment have diamond-shaped crosssections. Each of the kneading pins 40 extends from an inner peripherythereof along a respective radial direction of the housing module 20. Asshown in FIG. 5, the kneading pins 40 collectively define three kneadingpin columns 42 a, 42 b, and 42 c, each spaced 120° from each other aboutthe circumference of the screw 24 and dimensioned so as to be receivablein the gaps 34 (see FIG. 4). The kneading teeth 40 can be hollow andconnected to a supply means for permitting the injection of fluidconstituents through the kneading teeth and directly into the melt.

During operation, the screw 24 is rotated about its longitudinal axiswhile an axial stroke is superimposed on the screw 24 to oscillate thescrew 24 back and forth in the axial direction. A gear box (not shown),also available with the BUSS® Kneader brand kneader through GeorgFischer Plant Engineering, preferably ensures that each revolution ofthe screw 24 is accompanied by one full forward and backwards stroke ofthe screw 24. At the same time, the housing module 20 and kneading pins40 remain stationary relative to the rotating/oscillating screw 24.

The rotating/oscillating movement of the screw 24 causes the kneadingpins 40 to traverse across the faces of respective screw flights 30,thus generating a shear which cleans the faces of the screw flights 30and effects dispersion and distributive mixing. This relative movementbetween the screw flights 30 and the kneading pins 40 is explained belowin more detail with reference to FIG. 6, which shows selected pins 40 aand 40 c and their respective paths of movement relative to the screw24. As shown in FIG. 6, the kneading pins 40 move across the faces ofthe screw flights 30 and across the gaps 34, thereby cleaning the facesof the screw flights 30 and causing dispersion and distributive mixingto take place.

As mentioned above, a BUSS® Kneader model MDK/E-46 having a 46 mm singlescrew with a process L:D ratio of 11:1 can be used. This model ofkneader can be used in combination with a Reliance 40 HP 1750 rpm DCMotor and Flex Pak 3000 controller.

Vertical feeds can be provided at different axial locations along thelength of the housing module 20. Preferably, the inlet feeders arejacketed vertical screw feeders. Generally, the polymeric ingredientsand carbon fiber are introduced into the most upstream feed, fireretardants and other additives are added further downstream (along theaxial direction of the housing module 20), and the curing package isintroduced at the most downstream feed port. In this manner, theinsulation composition may be continuously produced. The temperature ofthe chamber is generally set in the range of from about 66° C. (150° F.)to 93° C. (200° F.) during operation.

An advantage of using the kneader of this second embodiment is that theinsulation composition discharged from the kneader can be introduceddirectly into an extruder for extrusion of the EPDM material. A suitableextruder for use with the kneader of this second embodiment is adischarge extruder GS70. The ability to extrude in this embodimentprovides improvements over conventional techniques, in which theinsulation composition is calendered into sheets and then cut.

EXAMPLES

The following examples illustrate embodiments which have been made inaccordance with the present invention. Also set forth are examplesprepared for comparison purposes. The inventive embodiments are notexhaustive or exclusive, but merely representative of the many types ofembodiments which may be prepared according to this invention.

TABLE I (all units in parts by weight) COMPARATIVE EXAMPLE EXAMPLEIngredient 1 2 3 4 5 6 A B TRILENE ® 67 [liquid 100 100 100 100 100 10040 EPDM] DSM KELTAN ® [solid 50 EPDM] NORDEL ® 1040 [solid 80 EPDM]Neoprene FB 20 [plasticizer] HYPALON ® 20 10 [polymer] PANEX ® 33 × 8[¼″ 26 fibers] FORTAFIL ® 144 38.5 45 40.5 30 25.7 26.85 [carbon fibers]VCM [carbon fibers] 41 AKROCHEM ® P-133 5 [plasticizer/tackifier]AGERITE ® Stalite S 2 2 2 2 2 2 2 1 [antioxidant] AGERITE ® Resin D 2[antioxidant] HI-SIL ® 233 [fire 5 5 5 5 5 5 3 retardant filler/char-forming agent] FIREBRAKE ® ZB [fire 19.5 retardant filler/char-formingagent] Hydral 710 Aluminum 19.5 Trihydrate [filler] Carbon black[filler] 1 DC4-7051 [fire 5 8.5 5 5 5 retardant/char-forming agent]Ferric oxide 1.13 [filler/pigment] Antimony oxide (4% 18 DIDP) [fireretardant/filler] Dechlorane Plus ® 25 45 [fire retardantfiller/char-forming agent] Pentaerythritol PE 200 8.5 [fire retardantfiller/char-forming agent] PHOS-CHECK ® P/40 30 [fire retardantfiller/char-forming agent] Melamine cyanurate [fire 25 25 25 retardantfiller/char-forming agent] Zinc oxide [activator] 5 5 Kadox 920C zincoxide 5 4 4 4 4 5 [activator] ALTAX ® [accelerator] 1.1 1.5 1.5 1.5 1.51.2 Accelerator VS 2.7 AKROFORM ® 0.3 0.3 0.3 0.3 0.25 DELTA P.M.[accelerator] SULFADS ® 0.82 0.82 0.82 0.82 0.82 0.75 [accelerator]Butyl zimate 0.5 [accelerator] RHENOCURE ® AP-5 3.5 [accelerator]CRYSTEX ® OT-20 1.05 1.22 1.22 1.22 1.22 1.22 [curative] SP-1056[curative] 1.1 CAPTAX ® 1 [accelerator] TELLURAC ® 0.5 [accelerator]Sulfur [curative] 1 Total Parts by Weight 172.8 192.3 223.9 202.3 149.8171.2 178.95 153.25

Examples 1 and 6

All solid ingredients, with the exception of the TRILENE® 67, wereblended in a V-shell blender at ambient temperature over several hours.The TRILENE® 67 was separately introduced into a Brabender mixerequipped with a sigma blade operating at 10 rpm and set to 60° C.(140°F.). The TRILENE® 67 was mixed in the Brabender mixer for asufficient amount of time to warm the TRILENE® 67 to 60°C. Next, theblended material from the V-shell blender was introduced into theBrabender mixer and allowed to mix with the TRILENE® 67 until the fiberswere uniformly dispersed in the TRILENE® 67. The formulation was thendumped from the Brabender mixer to a mill for shaping into sheets beforecooling. Each sheet was about 1.27 cm (0.5 inch) in thickness.

Examples 2 Through 5

All solid ingredients, with the exception of the TRILENE® 67 and thecarbon fibers, were blended in a V-shell blender at ambient temperatureover several hours. The TRILENE® 67 was separately introduced into aBrabender mixer equipped with a sigma blade operating at 50 rpm and setto 77° C. (170°F.). The TRILENE® 67 was mixed in the Brabender mixer fora sufficient amount of time to warm the TRILENE® 67 to 77°C. Next, theblended material from the V-shell blender was introduced into theBrabender mixer and allowed to mix with the TRILENE® 67. The speed ofthe Brabender mixer was then slowed to 20 rpm, and the fibers wereintroduced into the Brabender mixer and mixed until the fibers wereuniformly dispersed in the TRILENE® 67. The formulation was then dumpedfrom the Brabender mixer to a mill for shaping into sheets beforecooling. Each sheet was about 1.27 cm (0.5 inch) in thickness.

Comparative Examples A and B

Comparative Example A was prepared by mixing all of the ingredients,with the exception of the carbon fiber, in a laboratory mixer. Thecarbon fiber was incorporated into this mixture in a twin screw extruder(containing counter-rotating screws) by adding the mixed polymericmaterial and the carbon fibers in a single port of the twin screwextruder. Comparative Example B was made by solvent processing with ahydrocarbon solvent.

TABLE II Comparative EXAMPLE Example 1 2 3 4 5 6 A B Average AblationRate for Lower 3.29 3.98 4.89 3.96 4.05 3.31 3.37 3.45 Section (mm/s)Average Ablation Rate for Middle 12.34 9.36 12.63 9.97 12.97 11.80 16.9612.76 Section (mm/s) Average Ablation Rate for Upper 23.44 14.12 12.7212.79 20.45 17.94 35.23 18.13 Section (mm/s)

From Table II, it is seen that the inventive examples containing liquidEPDM as their exclusive EPDM ingredient (i.e., no solid EPDM) exhibitedcomparative and, in some instances, improved ablative properties toComparative Example A (containing less than half liquid EPDM based onthe total weight of EPDM ingredients) and Comparative Example B(containing no liquid EPDM).

Examples 7 through 9 were prepared in accordance with a secondembodiment of this invention by kneading the insulation in a BUSS®Kneader. The ingredients of the insulation compositions of Examples 7through 9 are set forth below in Table III. The ablative properties ofExamples 7 through 9, a comparison of these properties to that ofinventive Example 4, are set forth below in Table IV.

TABLE III (all units in parts by weight) EXAMPLE Ingredient 7 8 9 DSMKELTAN ® 1446A [solid 100 100 100 EPDM] FORTAFIL ® 243 [carbon fibers]40.52 44.45 55.55 Cure/Filler 14.86 14.88 14.88 Fire Retardant 47.0262.72 51.62 Total Parts by Weight 202.40 222.05 222.05

TABLE IV EXAMPLE 7 8 9 4 Average Ablation Rate for Lower 3.68 2.85 3.443.35 Section (mm/s) Average Ablation Rate for Middle 9.41 8.97 9.79 9.40Section (mm/s) Average Ablation Rate for Upper 15.42 14.75 13.61 11.64Section (mm/s)

As shown by Table IV, the insulation prepared in a BUSS® Kneader withoutany liquid EPDM exhibited comparable erosion resistance to Example 4,which was prepared in a sigma mixer with liquid EPDM.

The tests were performed in a char motor, such as the one illustrated inFIG. 7. Char motors are constructed to evaluate the ablative propertiesof solid rocket motor case insulation materials. A char motor includes apropellant beaker 70 to provide the combustion gases, evaluationchambers to hold the test materials, and a constricting nozzle toproduce the required pressure. The evaluation chamber is divided intothree sections. The first one is a “low-velocity” cylindrical region 72about eight inches long and eight inches in diameter (approximately thesame diameter as the propellant beaker 70). A short conical transitionchamber 74 constricts the gas flow into a diameter of about two inchesand vents the propellant gases into a 22-inch long conical test chamber.This test chamber is divided into the “mid-velocity” region 76 and“high-velocity” region 78.

Samples of insulation material to be evaluated are molded, cured, andbonded with epoxy into each of the test chambers. Prior to assembly, thecured length is determined and the thickness of each evaluation materialis measured at selected intervals, nominally one inch apart. Each sampleis also weighed. The samples are then assembled into the low-velocitysection, the mid-velocity section, and the high-velocity section. Afterfiring, the motor is disassembled and each sample is measured again. Theablation rate is determined by subtracting the post-fired thickness ofthe insulation (i.e., after the char had been removed) at a given pointfrom the prefired thickness and dividing the result by the bum time ofthe motor. For these tests, more than one section of material wasmeasured, and the average of all of the sections is reported above.

The char motors were fired with RSRM TP-H1148 (polybutadieneacrylic acidacrylonitrile (PBAN-based)) propellant. For Examples 1 through 3 andcomparative Example A set forth in Tables I and II, the motor was firedfor 12.10 seconds at an average pressure of 880 psi. For Examples 4through 6 set forth in Tables I and II, the motor was fired for 11.56seconds at an average pressure of 825 psi. Comparative Example B wasfired for 11.89 seconds at an average pressure of 885 psi. For Examples7 through 9 and 4 set forth in Tables III and IV, the motor was firedfor 12.46 seconds at an average pressure of 916.07 psi.

The foregoing detailed description of the invention has been providedfor the purpose of explaining the principles of the invention and itspractical application, thereby enabling others skilled in the art tounderstand the invention for various embodiments and with variousmodifications as are suited to the particular use contemplated. Thisdescription is not intended to be exhaustive or to limit the inventionto the precise embodiments disclosed. Modifications and equivalents willbe apparent to practitioners skilled in this art and are encompassedwithin the spirit and scope of the appended claims.

1. A method of insulating a case of a rocket motor loaded with a solidpropellant, comprising: providing a kneader comprising at least onehousing module provided with a chamber, a plurality of kneader pinsextending radially into the chamber from the at least one housingmodule, and a screw which is rotatable about a longitudinal axis thereofand movable along the longitudinal axis in a reciprocating motion, thescrew having a peripheral surface including a plurality of screw flightsarranged to collectively define a discontinuous screw thread extendingalong the longitudinal axis of the screw; preparing insulation from acomposition comprising at least one cross-linkable polymer and carbonfibers by introducing the composition into the kneader and rotating thescrew along the longitudinal axis thereof while superimposing thelongitudinal reciprocating motion to the screw under substantiallysolvent-free conditions; curing the composition to form the insulation;and insulating a case of a rocket motor with the insulation.
 2. A methodof insulating a case of a rocket motor loaded with a solid propellant,comprising: providing a kneader comprising a plurality of kneader pinsextending radially into a chamber of at least one housing module of thekneader and a screw, the screw rotatable about a longitudinal axisthereof and movable along the longitudinal axis in a reciprocatingmotion and the screw having a peripheral surface including a pluralityof screw flights arranged to collectively define a discontinuous screwthread extending along the longitudinal axis of the screw; preparinginsulation from a composition comprising at least one cross-linkableethylene propylene diene monomer (“EPDM”) terpolymer and carbon fibersby rotating the screw along the longitudinal axis thereof whilesuperimposing an axially reciprocating motion to the screw to mix thecomposition under substantially solvent-free conditions; curing thecomposition to form the insulation; and insulating a case of a rocketmotor with the insulation.
 3. The method of claim 2, wherein insulatinga case of a rocket motor comprises applying the insulation to aninterior surface of the case and interposing the insulation between theinterior surface and a solid propellant.
 4. The method of claim 2,wherein preparing insulation from a composition comprising at least onecross-linkable EPDM terpolymer and carbon fibers comprises preparing theinsulation from a composition comprising 100 weight percent of across-linkable liquid EPDM terpolymer and the carbon fibers.
 5. Themethod of claim 2, wherein preparing insulation from a compositioncomprising at least one cross-linkable EPDM terpolymer and carbon fiberscomprises preparing the insulation from a composition comprising atleast about 90 weight percent of a cross-linkable liquid EPDM terpolymerand the carbon fibers.
 6. The method of claim 2, wherein preparinginsulation from a composition comprising at least one cross-linkableEPDM terpolymer and carbon fibers comprises preparing the insulationfrom a composition comprising at least about 50 weight percent of across-linkable liquid EPDM terpolymer and the carbon fibers.
 7. Themethod of claim 2, wherein preparing insulation from a compositioncomprising at least one cross-linkable EPDM terpolymer and carbon fiberscomprises preparing the insulation from a composition that is free of across-linkable liquid EPDM terpolymer.
 8. The method of claim 2, whereinpreparing insulation from a composition comprising at least onecross-linkable EPDM terpolymer and carbon fibers comprises preparing thecomposition from dry ingredients under conditions in which thecomposition comprises not more than about 5 weight percent of volatilesolvent based on the dry ingredients in the composition.
 9. The methodof claim 2, wherein preparing insulation from a composition comprisingat least one cross-linkable EPDM terpolymer and carbon fibers comprisesmixing the composition in the kneader in the absence of any volatilesolvent.
 10. A method of insulating a case of a rocket motor loaded witha solid propellant, comprising: providing a kneader comprising achamber, a plurality of kneader pins extending radially into thechamber, and a screw which is rotatable about a longitudinal axisthereof and movable along the longitudinal axis in a reciprocatingmotion, the screw having a peripheral surface comprising a plurality ofscrew flights arranged to collectively define a discontinuous screwthread extending along the longitudinal axis of the screw; rotating thescrew along the longitudinal axis thereof while superimposing an axial,oscillating motion to the screw to mix a composition comprising at leastone cross-linkable polymer and carbon fibers in the kneader undersubstantially solvent-free conditions; curing the composition to forminsulation; and insulating a case of a rocket motor with the insulation.11. The method of claim 1, wherein preparing insulation from acomposition comprising at least one cross-linkable polymer and carbonfibers comprises preparing the insulation from a composition comprisingcarbon fibers including a carbon content of greater than about 98 weightpercent.